Thursday, October 30, 2014

Direct Measurement of the Cortical Tension during the Growth of Membrane Blebs

Julia Peukes, Timo Betz

Mechanics is at the heart of many cellular processes and its importance has received considerable attention during the last two decades. In particular, the tension of cell membranes, and more specifically of the cell cortex, is a key parameter that determines the mechanical behavior of the cell periphery. However, the measurement of tension remains challenging due to its dynamic nature. Here we show that a noninvasive interferometric technique can reveal time-resolved effective tension measurements by a high-accuracy determination of edge fluctuations in expanding cell blebs of filamin-deficient melanoma cells. The introduced technique shows that the bleb tension is ∼10–100 pN/μm and increases during bleb growth. Our results directly confirm that the subsequent stop of bleb growth is induced by an increase of measured tension, possibly mediated by the repolymerized actin cytoskeleton.


Optical manipulation of biological particles using LP21 mode in fiber

Shijie Chen, He Huang, Hongmei Zou, Qing Li, Jian Fu, Feng Lin and X Wu

We demonstrate the optical manipulation of biological particles using a low-order LP21 fiber mode. The focused four-lobed LP21 mode distribution was theoretically and experimentally found to be effective in optical tweezer applications, including selective cellular pick-up, pairing, grouping or separation, as well as rotation of cell dimers and clusters. Our proposed theoretical model estimates both the translational dragging force and rotational torque in good accordance with experimental data. With a simple all-fiber configuration, and low peak irradiation to target bioparticles, the proposed LP21 'optical chuck' system has great application potential in biological test systems.


Radiation forces on Rayleigh particles using a focused anomalous vortex beam under paraxial approximation

Dongjie Zhang, Yuanjie Yang

The radiation forces of focused anomalous vortex beams acting upon a Rayleigh dielectric sphere are studied theoretically and numerically, based on the Rayleigh scattering theory, within the framework of paraxial approximation. It is shown that a focused anomalous vortex beam with a suitable mode order and topological charge can be used to trap and manipulate a dielectric sphere whose refractive index is smaller or bigger than the ambient one at the focal point. The influences of the topological charges and the beam orders on the radiation force are also discussed. Furthermore, the stability conditions for effective trapping the Rayleigh particles are analyzed.


Localized Opto-Mechanical Control of Protein Adsorption onto Carbon Nanotubes

Dakota O'Dell, Xavier Serey, Pilgyu Kang & David Erickson

Chemical reactions can be described by an energy diagram along a reaction coordinate in which an activation barrier limits the rate at which reactants can be transformed into products. This reaction impedance can be overcome by reducing the magnitude of the barrier through the use of catalysis, increasing the thermal energy of the system, or through macroscopic mechanical processes. Here, we demonstrate direct molecular-scale control of a reaction through the precise application of opto-mechanical work. The method uses optical gradient forces generated in the evanescent field surrounding hybrid photonic-plasmonic structures to drive an otherwise unlikely adsorption reaction between proteins and carbon nanotubes. The adsorption of immunoglobulins on carbon nanotubes is used as a model reaction and investigated with an extended DLVO theory. The technique is also used to force a Förster resonance energy transfer between fluorophores on mismatched immunoglobulin proteins and is expected to lead to novel forms of chemical synthesis.


A long-range polarization-controlled optical tractor beam

Vladlen Shvedov, Arthur R. Davoyan, Cyril Hnatovsky, Nader Engheta & Wieslaw Krolikowski

The laser beam has become an indispensable tool for the controllable manipulation and transport of microscopic objects in biology, physical chemistry and condensed matter physics. In particular, ‘tractor’ laser beams can draw matter towards a laser source and perform, for instance, all-optical remote sampling. Recent advances in lightwave technology have already led to small-scale experimental demonstrations of tractor beams1, 2, 3, 4. However, the realization of long-range tractor beams has not gone beyond the realm of theoretical investigations5, 6, 7, 8, 9. Here, we demonstrate the stable transfer of gold-coated hollow glass spheres against the power flow of a single inhomogeneously polarized laser beam over tens of centimetres. Additionally, by varying the polarization state of the beam we can stop the spheres or reverse the direction of their motion at will.


Long-term influence of fluid inertia on the diffusion of a Brownian particle

Giuseppe Pesce, Giorgio Volpe, Giovanni Volpe, and Antonio Sasso

We experimentally measure the effects of fluid inertia on the diffusion of a Brownian particle at very long time scales. In previous experiments, the use of standard optical tweezers introduced a cutoff in the free diffusion of the particle, which limited the measurement of these effects to times comparable with the relaxation time of the fluid inertia, i.e., a few milliseconds. Here, by using blinking optical tweezers, we detect these inertial effects on time scales several orders longer up to a few seconds. The measured mean square displacement of a freely diffusing Brownian particle in a liquid shows a deviation from the Einstein-Smoluchowsky theory that diverges with time. These results are consistent with a generalized theory that takes into account not only the particle inertia but also the inertia of the surrounding fluid.


Sister kinetochores are mechanically fused during meiosis I in yeast

Krishna K. Sarangapani, Eris Duro, Yi Deng, Flavia de Lima Alves, Qiaozhen Ye, Kwaku N. Opoku, Steven Ceto, Juri Rappsilber, Kevin D. Corbett, Sue Biggins, Adèle L. Marston, Charles L. Asbury

Production of healthy gametes requires a reductional meiosis I division in which replicated sister chromatids comigrate, rather than separate as in mitosis or meiosis II. Fusion of sister kinetochores during meiosis I may underlie sister chromatid comigration in diverse organisms, but direct evidence for such fusion has been lacking. We used laser trapping and quantitative fluorescence microscopy to study native kinetochore particles isolated from yeast. Meiosis I kinetochores formed stronger attachments and carried more microtubule-binding elements than kinetochores isolated from cells in mitosis or meiosis II. The meiosis I–specific monopolin complex was both necessary and sufficient to drive these modifications. Thus, kinetochore fusion directs sister chromatid comigration, a conserved feature of meiosis that is fundamental to Mendelian inheritance.


Tuesday, October 28, 2014

Direct measurement of cell protrusion force utilizing a robot-aided cell manipulation system with optical tweezers for cell migration control

Xue Gou, Hao Yang, Tarek M Fahmy, Yong Wang, Dong Sun

Cell migration refers to the directional cell movement in response to a chemoattractant gradient, a key process that occurs in a wide variety of biological phenomena. Cell protrusion force is generated by the actin polymerization of a cell, which drives the cell to move toward the stimulus as induced by the chemoattractant gradient. This paper presents a new methodology for the direct measurement of cell protrusion force utilizing a robot-aided optical tweezer system. The functionalized beads that are robotically trapped and placed near the cell serve as both cell migration stimulators and protrusion force probes. The force generated by the actin polymerization of the cell propels the bead to move away from the trapping center when the cell comes in contact with the bead. Such a deviation can be determined and used to calculate the trapping force, which is equal to the protrusion force at a balanced position. With the quantitative measurement of the protrusion, we find that the protrusion force of a live cell in response to a chemoattractant within the range of hundreds of piconewtons. We further probe the protrusion force distribution at the cell leading edge and find that the highest protrusion force appears at the cell migration direction. These measurements can help us characterize the mechanism of cell migration and lay a solid foundation for further proactive control of cell movement.


Highly cooperative stress relaxation in two-dimensional soft colloidal crystals

Berend van der Meer, Weikai Qi, Remco G. Fokkink, Jasper van der Gucht, Marjolein Dijkstra, and Joris Sprakel

Stress relaxation in crystalline solids is mediated by the formation and diffusion of defects. Although it is well established how externally generated stresses relax, through the proliferation and motion of dislocations in the lattice, it remains relatively unknown how crystals cope with internal stresses. We investigate, both experimentally and in simulations, how highly localized stresses relax in 2D soft colloidal crystals. When a single particle is actively excited, by means of optical tweezing, a rich variety of highly collective stress relaxation mechanisms results. These relaxation processes manifest in the form of open strings of cooperatively moving particles through the motion of dissociated vacancy-interstitial pairs, and closed loops of mobile particles, which either result from cooperative rotations in transiently generated circular grain boundaries or through the closure of an open string by annihilation of a vacancy-interstitial pair. Surprisingly, we find that the same collective events occur in crystals that are excited by thermal fluctuations alone; a large thermal agitation inside the crystal lattice can trigger the irreversible displacements of hundreds of particles. Our results illustrate how local stresses can induce large-scale cooperative dynamics in 2D soft colloidal crystals and shed light on the stabilization mechanisms in ultrasoft crystals.


Two properties of twisted-light absorption

Andrei Afanasev, Carl E. Carlson, and Asmita Mukherjee

We discuss two features of twisted-light absorption both by hydrogen-like atoms and by microparticles. First, we extend the treatment of atomic photoexcitation by twisted photons to include atomic recoil, derive generalized quantum selection rules, and consider phenomena of forbidden atomic transitions. In particular, we demonstrate that whatever part of the twisted photon’s total helicity does not go into exciting the atomic state goes into making the atom as a whole revolve about the photon’s symmetry axis. Second, using the same electromagnetic potential for the twisted light beams, we analyze the radiation pressure from these beams on micro-sized particles and verify that while the Poynting vector can in some circumstances point back toward the source, a complete analysis nonetheless gives a repulsive radiation pressure.


Plasmon-induced Lorentz forces of nanowire chiral hybrid modes

Matthew Moocarme, Benjamin Kusin, and Luat T. Vuong

The mechanical forces associated with surface currents are widely overlooked and point to a new family of plasmonically-driven processes. Here, we investigate the Lorentz forces acting on a free electron gas that is bound to the surface of a nanowire. We demonstrate that appreciable mechanical forces are produced by longer illumination wavelengths between longitudinal and transverse absorption resonances via the excitation of chiral hybrid plasmon modes. We are the first to associate plasmonic activity as the underlying mechanism for nanowire rotation, which explains prior experimental results. The presence of chiral hybrid plasmon modes yields the greatest net translation and torque forces. The asymmetric plasmon behavior subsequently affects the complex nonlinear dynamics of plasmonic nonspherical nanoparticles in fluids.


Rotating Au nanorod and nanowire driven by circularly polarized light

Jiunn-Woei Liaw, Ying-Syuan Chen, and Mao-Kuen Kuo

The wavelength-dependent optical torques provided by a circularly polarized (CP) plane wave driving Au nanorod (NR) and nanowire (NW) to rotate constantly were studied theoretically. Using the multiple multipole method, the resultant torque in terms of Maxwell’s stress tensor was analyzed. Numerical results show that the optical torque spectrum is in accordance with the absorption spectrum of Au NR/NW. Under the same fluence, the maximum optical torque occurs at the longitudinal surface plasmon resonance (LSPR) of Au NR/NW, accompanied by a severe plasmonic heating. The rotation direction of the light-driven NR/NW depends on the handedness of CP light. In contrast, the optical torque exerted on Au NR/NW illuminated by a linearly polarized light is null at LSPR. Due to the plasmonic effect, the optical torque on Au NR/NW by CP light is two orders of magnitude larger than that on a dielectric NR/NW of the same size. The steady-state rotation of NR/NW in water, resulting from the balance of optical torque and viscous torque, was also discussed. Our finding shed some light on manipulating a CP light-driven Au NR/NW as a rotating nanomotor for a variety of applications in optofluidics and biophysics.


The Endoplasmic reticulum: A dynamic and well connected organelle

Chris Hawes, Petra Kiviniemi and Verena Kriechbaumer

The endoplasmic reticulum forms the first compartment in a series of organelles which comprise the secretory pathway. It takes the form of an extremely dynamic and pleomorphic membrane bounded network of tubules and cisternae which have numerous different cellular functions. In this review we discuss the nature of endoplasmic reticulum structure and dynamics, its relationship with closely associated organelles, and its possible function as a highway for the distribution and delivery of a diverse range of structures from metabolic complexes to viral particles.


Chiral discrimination in optical trapping and manipulation

David S Bradshaw and David L Andrews

When circularly polarized light interacts with chiral molecules or nanoscale particles powerful symmetry principles determine the possibility of achieving chiral discrimination, and the detailed form of electrodynamic mechanisms dictate the types of interaction that can be involved. The optical trapping of molecules and nanoscale particles can be described in terms of a forward-Rayleigh scattering mechanism, with trapping forces being dependent on the positioning within the commonly non-uniform intensity beam profile. In such a scheme, nanoparticles are commonly attracted to local potential energy minima, ordinarily towards the centre of the beam. For achiral particles the pertinent material response property usually entails an electronic polarizability involving transition electric dipole moments. However, in the case of chiral molecules, additional effects arise through the engagement of magnetic counterpart transition dipoles. It emerges that, when circularly polarized light is used for the trapping, a discriminatory response can be identified between left- and right-handed polarizations. Developing a quantum framework to accurately describe this phenomenon, with a tensor formulation to correctly represent the relevant molecular properties, the theory leads to exact analytical expressions for the associated energy landscape contributions. Specific results are identified for liquids and solutions, both for isotropic media and also where partial alignment arises due to a static electric field. The paper concludes with a pragmatic analysis of the scope for achieving enantiomer separation by such methods.


Monday, October 27, 2014

Simple technique for generating the perfect optical vortex

Joaquín García-García, Carolina Rickenstorff-Parrao, Rubén Ramos-García, Víctor Arrizón, and Andrey S. Ostrovsky

We propose an improved technique for generating the perfect optical vortex. This technique is notable for the simplicity of its practical realization and high quality of the results. The efficiency of the proposed technique is illustrated with the results of physical experiments and an example of its application in optical trapping of small particles.


Photothermal Heating Enabled by Plasmonic Nanostructures for Electrokinetic Manipulation and Sorting of Particles

Justus Chukwunonso Ndukaife, Avanish Mishra, Urcan Guler, Agbai George Agwu Nnanna, Steven T. Wereley, and Alexandra Boltasseva

Plasmonic nanostructures support strong electromagnetic field enhancement or optical “hot spots” that are accompanied by local heat generation. This heating effect is generally seen as an obstacle to stable trapping of particles on a plasmonic substrate. In this work, instead of treating the heating effect as a hindrance, we utilized the collective photoinduced heating of the nanostructure array for high-throughput trapping of particles on a plasmonic nanostructured substrate. The photoinduced heating of the nanostructures is combined with an ac electric field of less than 100 kHz, which results in creation of a strong electrothermal microfluidic flow. This flow rapidly transports suspended particles toward the plasmonic substrate, where they are captured by local electric field effects. This work is envisioned to have application in biosensing and surface-enhanced spectroscopies such as SERS.


A model colloidal gel for coordinated measurements of force, structure, and rheology

Lilian C. Hsiao, Michael J. Solomon, Kathryn A. Whitaker and Eric M. Furst

We introduce a model gel system in which colloidal forces, structure, and rheology are measured by balancing the requirements of rheological and microscopy techniques with those of optical tweezers. Sterically stabilized poly(methyl methacrylate) colloids are suspended in cyclohexane (CH) and cyclohexyl bromide (CHB) with dilute polystyrene serving as a depletion agent. A study of the optical trap strength, rheology, and microscopic structure of the gels as a function of CH/CHB solvent composition identifies the conditions for which these measurements can be applied to characterize gel properties. The results indicate that a solvent comprising 37% weight fraction CH (wCH  = 0.37) provides sufficient refractive index contrast to enable optical trapping, while maintaining good confocal microscopy imaging quality and minimal sedimentation effects on the bulk rheology. At this condition, and at a depletant concentration c = 8.64 mg/ml (c/c* = 0.81), results from optical trapping in a dilute sample show that 50% of bonds rupture at (3.3 ± 0.5) pN. The linear strain-dependent elastic modulus of the corresponding gel (ϕ = 0.20) is G′ = (1.8 ± 0.6) Pa, and the mean contact number of the colloids in the gel structure is 〈z〉 = 5.4 ± 0.1. The development of this model colloidal gel system yields a concomitant characterization of the interparticle forces, microstructure, and bulk rheology in a single experimental system, thereby allowing the simultaneous comparison of these different measures.


High-throughput single-molecule studies of protein–DNA interactions

Aaron D. Robison, Ilya J. Finkelstein

Fluorescence and force-based single-molecule studies of protein–nucleic acid interactions continue to shed critical insights into many aspects of DNA and RNA processing. As single-molecule assays are inherently low-throughput, obtaining statistically relevant datasets remains a major challenge. Additionally, most fluorescence-based single-molecule particle-tracking assays are limited to observing fluorescent proteins that are in the low-nanomolar range, as spurious background signals predominate at higher fluorophore concentrations. These technical limitations have traditionally limited the types of questions that could be addressed via single-molecule methods. In this review, we describe new approaches for high-throughput and high-concentration single-molecule biochemical studies. We conclude with a discussion of outstanding challenges for the single-molecule biologist and how these challenges can be tackled to further approach the biochemical complexity of the cell.


Optical Tweezers: Autonomous Robots for the Manipulation of Biological Cells

Banerjee, A. ; Chowdhury, S. ; Gupta, S.K.

Optical tweezers (OTs) are a popular tool for manipulating biological objects, especially cells [1], [2]. Using a tightly focused laser beam, they exert sufficient forces to tweeze, i.e., hold (trap) and move, freely diffusing cells in the vicinity of the beam focus. The beam can be focused at any point in the workspace, which is typically a liquid-filled glass slide. The trapped cell can, thus, be translated and rotated (transported) in three dimensions by changing the beam focus position. OTs provide certain advantages over other cell-manipulation techniques. They are able to manipulate cells with a greater degree of precision as compared with microfluidic flow. Significant contact forces are not exerted on the cells, unlike in mechanical manipulation, thereby avoiding damages due to contact friction or surface chemistry. The cells are also easily released at the end of the manipulation by simply switching off the laser beam. Hence, OTs have been extensively used for mechanical characterization of cells by measuring their viscoelastic properties to distinguish between normal and diseased cells [3]. They have also been used for separating cells of different types [4] and investigating the response of cells to external stimuli [5]. However, manual or teleoperated control of the laser beam has limited their applicability for multicellular studies.


Bessel beams from semiconductor light sources

G.S. Sokolovskii, V.V. Dudelev, S.N. Losev, K.K. Soboleva, A.G. Deryagin, K.A. Fedorova, V.I. Kuchinskii, W. Sibbett, E.U. Rafailov

We report on recent progress in the generation of non-diffracting (Bessel) beams from semiconductor light sources including both edge-emitting and surface-emitting semiconductor lasers as well as light-emitting diodes (LEDs). Bessel beams at the power level of Watts with central lobe diameters of a few to tens of micrometers were achieved from compact and highly efficient lasers. The practicality of reducing the central lobe size of the Bessel beam generated with high-power broad-stripe semiconductor lasers and LEDs to a level unachievable by means of traditional focusing has been demonstrated. We also discuss an approach to exceed the limit of power density for the focusing of radiation with high beam propagation parameter M2. Finally, we consider the potential of the semiconductor lasers for applications in optical trapping/tweezing and the perspectives to replace their gas and solid-state laser counterparts for a range of implementations in optical manipulation towards lab-on-chip configurations.


Thursday, October 23, 2014

Pairwise Interactions of Colloids in Two-dimensional Geometric Confinement

Bum Jun Park, Bomsock Lee and Taekyung Yu

We present the pairwise interaction behaviour of colloids confined in two-dimensional (2D) colloidal cages using optical laser tweezers. A single probe particle inside the hexagonal cage particles at a planar oil-water interface is allowed to diffuse freely and the spring constant is extracted from its trajectories. To evaluate the effect of multibody interactions, the pair interactions between the probe particle and each cage particle are directly measured by using optical tweezers. Based on pairwise additivity, Monte Carlo simulations are used to compare the values of the spring constant obtained from experiments and simulations. We find that the multibody interactions negligibly occur, and thus the particle interactions confined in such colloidal cages are highly pairwise. This work demonstrates that the use of the pairwise assumption in numerical simulations is rational when interparticle repulsive interactions are sufficiently strong, such as the particle interactions at fluid-fluid interfaces.


Stability and interaction forces of oil-in-water emulsions as observed by optical tweezers – a proof-of-concept study

Julie Nilsen-Nygaard, Marit Sletmoen and Kurt Ingar Draget

Increased insight into the interactions occurring between emulsion droplets is important to a range of applications from the food and pharmaceutical industries to oil recovery and mineral flotation. These interactions are often modified by the adsorption at the oil–water interface of surface-active species such as small molecule surfactants, proteins or polymers, in order to meet functional requirements of the emulsions. However, the experimental challenges faced when attempting to study these forces acting between emulsion droplets have hampered the progress in the understanding of the fundamental forces and to which extent these forces influence the destabilizing processes. In this paper we describe emulsion droplet studies by applying optical tweezers. By capturing two emulsion droplets in separate optical traps and bringing them into proximity, the forces acting between them can be measured as a function of separation distance. In this proof-of-concept study the force versus distance curves of emulsion droplets of different stabilization was obtained. Focus has been placed on the relative differences between micro- and macromolecular stabilization of emulsion droplets. Effects on depletion interaction, relaxation behaviour of the interfacial polymer layer during compression of the droplets and electrostatic screening have been observed. The present article documents the suitability of optical tweezers in studies aiming at revealing the forces acting between individual emulsion droplets as well as limiting factors of the technology.


Chemical characterization of single micro- and nano-particles by optical catapulting–optical trapping–laser-induced breakdown spectroscopy

Francisco J. Fortes, Angel Fernández-Bravo, J. Javier Laserna

Spectral identification of individual micro- and nano-sized particles by the sequential intervention of optical catapulting, optical trapping and laser-induced breakdown spectroscopy is presented. The three techniques are used for different purposes. Optical catapulting (OC) serves to put the particulate material under inspection in aerosol form. Optical trapping (OT) permits the isolation and manipulation of individual particles from the aerosol, which are subsequently analyzed by laser-induced breakdown spectroscopy (LIBS). Once catapulted, the dynamics of particle trapping depends both on the laser beam characteristics (power and intensity gradient) and on the particle properties (size, mass and shape). Particles are stably trapped in air at atmospheric pressure and can be conveniently manipulated for a precise positioning for LIBS analysis. The spectra acquired from the individually trapped particles permit a straightforward identification of the material inspected. Variability of LIBS signal for the inspection of Ni microspheres was 30% relative standard deviation. OC–OT–LIBS permits the separation of particles in a heterogeneous mixture and the subsequent analysis of the isolated particle of interest. In order to evaluate the sensitivity of the approach, the number of absolute photons emitted by a single trapped particle was calculated. The limit of detection (LOD) for Al2O3 particles was calculated to be 200 attograms aluminium.


Determining Intrachain Diffusion Coefficients for Biopolymer Dynamics from Single-Molecule Force Spectroscopy Measurements

Michael T. Woodside, John Lambert, Kevin S.D. Beach

The conformational diffusion coefficient for intrachain motions in biopolymers, D, sets the timescale for structural dynamics. Recently, force spectroscopy has been applied to determine D both for unfolded proteins and for the folding transitions in proteins and nucleic acids. However, interpretation of the results remains unsettled. We investigated how instrumental effects arising from the force probes used in the measurement can affect the value of D recovered via force spectroscopy. We compared estimates of D for the folding of DNA hairpins found from measurements of rates and energy landscapes made using optical tweezers with estimates obtained from the same single-molecule trajectories via the transition path time. The apparent D obtained from the rates was much lower than the result found from the same data using transition time analysis, reflecting the effects of the mechanical properties of the force probe. Deconvolution of the finite compliance effects on the measurement allowed the intrinsic value to be recovered. These results were supported by Brownian dynamics simulations of the effects of force-probe compliance and bead size.


Graded-index optical fiber tweezers with long manipulation length

Yuan Gong, Wei Huang, Qun-Feng Liu, Yu Wu, Yunjiang Rao, Gang-Ding Peng, Jinyi Lang, and Ke Zhang
Long manipulation length is critical for optical fiber tweezers to enhance the flexibility of non-contact trapping. In this paper a long manipulation distance of more than 40 μm is demonstrated experimentally by the graded-index fiber (GIF) tweezers, which is fabricated by chemically etching a GIF taper with a large cone angle of 58°. The long manipulation distance is obtained by introducing an air cavity between the lead-in single mode fiber and the GIF as well as by adjusting the laser power in the existence of a constant background flow. The influence of the cavity length and the GIF length on the light distribution and the focusing length of the GIF taper is investigated numerically, which is helpful for optimizing the parameters to perform stable optical trapping. This kind of optical fiber tweezers has advantages including low-cost, easy-to-fabricate and easy-to-use.


Non-contact fiber-optical trapping of motile bacteria: dynamics observation and energy estimation

Hongbao Xin, Qingyuan Liu & Baojun Li

The dynamics and energy conversion of bacteria are strongly associated with bacterial activities, such as survival, spreading of bacterial diseases and their pathogenesis. Although different discoveries have been reported on trapped bacteria (i.e. immobilized bacteria), the investigation on the dynamics and energy conversion of motile bacteria in the process of trapping is highly desirable. Here, we report a non-contact optical trapping of motile bacteria using a modified tapered optical fiber. Using Escherichia coli as an example, both single and multiple motile bacteria have been trapped and manipulated in a non-contact manner. Bacterial dynamics has been observed and bacterial energy has been estimated in the trapping process. This non-contact optical trapping provides a new opportunity for better understanding the bacterial dynamics and energy conversion at the single cell level.


Gradient and scattering forces in photoinduced force microscopy

Junghoon Jahng, Jordan Brocious, Dmitry A. Fishman, Fei Huang, Xiaowei Li, Venkata Ananth Tamma, H. Kumar Wickramasinghe, and Eric Olaf Potma

A theoretical and experimental analysis of the dominant forces measured in photoinduced force microscopy is presented. It is shown that when operated in the noncontact and soft-contact modes, the microscope is sensitive to the optically induced gradient force (Fg) and the scattering force (Fsc). The reconstructed force-distance curve reveals a tip-dependent scattering force in the 30–60 pN range. Whereas the scattering force is virtually insensitive to the nanoscopic tip-sample distance, the gradient force shows a z−4 dependence and is manifest only for tip-sample distances of a few nm. Measurements on glass, gold nanowires, and molecular clusters of silicon naphtalocyanine confirm that the gradient force is strongly dependent on the polarizability of the sample, enabling spectroscopic imaging through force detection. The nearly constant Fsc and the spatially dependent Fg give rise to a complex force-distance curve, which varies from point to point in the specimen and dictates the image contrast observed for a given set point of the cantilevered tip.


Direct Quantification of Loop Interaction and π–π Stacking for G-Quadruplex Stability at the Submolecular Level

Chiran Ghimire, Soyoung Park, Keisuke Iida, Philip Yangyuoru, Haruka Otomo, Zhongbo Yu, Kazuo Nagasawa, Hiroshi Sugiyama, and Hanbin Mao

The well-demonstrated biological functions of DNA G-quadruplex inside cells call for small molecules that can modulate these activities by interacting with G-quadruplexes. However, the paucity of the understanding of the G-quadruplex stability contributed from submolecular elements, such as loops and tetraguanine (G) planes (or G-quartets), has hindered the development of small-molecule binders. Assisted by click chemistry, herein, we attached pulling handles via two modified guanines in each of the three G-quartets in human telomeric G-quadruplex. Mechanical unfolding using these handles revealed that the loop interaction contributed more to the G-quadruplex stability than the stacking of G-quartets. This result was further confirmed by the binding of stacking ligands, such as telomestatin derivatives, which led to similar mechanical stability for all three G-quartets by significant reduction of loop interactions for the top and bottom G-quartets. The direct comparison of loop interaction and G-quartet stacking in G-quadruplex provides unprecedented insights for the design of more efficient G-quadruplex-interacting molecules. Compared to traditional experiments, in which mutations are employed to elucidate the roles of specific residues in a biological molecule, our submolecular dissection offers a complementary approach to evaluate individual domains inside a molecule with fewer disturbances to the native structure.


The measurement of light momentum shines the path towards the cell

A. Farré, E. Martín-Badosa, M. Montes-Usategui

After an intense development of optical tweezers as a biophysical tool during the last decades, quantitative experiments in living cells have not found in this technique its best ally, due, in part, to the lack of a standard method to measure forces in complex environments. The existent alternatives either require complicated in situ calibrations, which make their use impossible in the study of dynamic processes, or they lack accuracy. Using an approach completely different from the most extended options, Steven Smith at Carlos Bustamante’s Lab at the University of Berkeley developed method based on the direct measurement of the momentum change of the trapping beam, which has the potential to become the standard for measuring forces in all kind of experiments. Measurements are performed regardless of the physical properties of the sample or the trapping laser, and they only depend on some parameters of the instrument design. As a consequence, the method does not need a continuous calibration and can be used in a wider range of experiments. However, its diffusion has been modest mainly because it requires a counter-propagating optical trapping system, which is difficult to implement and combine with other techniques. Here, we show how this method can be implemented in the more extended trapping configuration, optical tweezers, and how this relates to the well-known position detection technique, back-focal-plane interferometry. We finally discuss the potential of this method to perform experiments inside cells and also for commercial purposes, to make optical trapping available to non-experts.


Moving force of metal particle migration induced by laser irradiation in borosilicate glass: erratum

Hirofumi Hidai, Makoto Matsushita, Souta Matsusaka, Akira Chiba, and Noboru Morita

In our previous paper, we incorrectly calculated the total applied force on the metal particle. We have corrected the results and the affected figures. These corrections do not affect the conclusions of the published paper.


Visualization and quantification of nascent RAD51 filament formation at single-monomer resolution

Andrea Candelli, Jan Thomas Holthausen, Martin Depken, Ineke Brouwer, Mariëlla A. M. Franker, Margherita Marchetti, Iddo Heller, Stéphanie Bernard, Edwige B. Garcin, Mauro Modesti, Claire Wyman, Gijs J. L. Wuite, and Erwin J. G. Peterman

During recombinational repair of double-stranded DNA breaks, RAD51 recombinase assembles as a nucleoprotein filament around single-stranded DNA to form a catalytically proficient structure able to promote homology recognition and strand exchange. Mediators and accessory factors guide the action and control the dynamics of RAD51 filaments. Elucidation of these control mechanisms necessitates development of approaches to quantitatively probe transient aspects of RAD51 filament dynamics. Here, we combine fluorescence microscopy, optical tweezers, and microfluidics to visualize the assembly of RAD51 filaments on bare single-stranded DNA and quantify the process with single-monomer sensitivity. We show that filaments are seeded from RAD51 nuclei that are heterogeneous in size. This heterogeneity appears to arise from the energetic balance between RAD51 self-assembly in solution and the size-dependent interaction time of the nuclei with DNA. We show that nucleation intrinsically is substrate selective, strongly favoring filament formation on bare single-stranded DNA. Furthermore, we devised a single-molecule fluorescence recovery after photobleaching assay to independently observe filament nucleation and growth, permitting direct measurement of their contributions to filament formation. Our findings yield a comprehensive, quantitative understanding of RAD51 filament formation on bare single-stranded DNA that will serve as a basis to elucidate how mediators help RAD51 filament assembly and accessory factors control filament dynamics.


DNA Y Structure: A Versatile, Multidimensional Single Molecule Assay

James T. Inman, Benjamin Y. Smith, Michael A. Hall, Robert A. Forties, Jing Jin, James P. Sethna, and Michelle D. Wang

Optical trapping is a powerful single molecule technique used to study dynamic biomolecular events, especially those involving DNA and DNA-binding proteins. Current implementations usually involve only one of stretching, unzipping, or twisting DNA along one dimension. To expand the capabilities of optical trapping for more complex measurements would require a multidimensional technique that combines all of these manipulations in a single experiment. Here, we report the development and utilization of such a novel optical trapping assay based on a three-branch DNA construct, termed a “Y structure”. This multidimensional assay allows precise, real-time tracking of multiple configurational changes. When the Y structure template is unzipped under both force and torque, the force and extension of all three branches can be determined simultaneously. Moreover, the assay is readily compatible with fluorescence, as demonstrated by unzipping through a fluorescently labeled, paused transcription complex. This novel assay thus allows for the visualization and precision mapping of complex interactions of biomechanical events.


Rapid interrogation of the physical and chemical characteristics of salbutamol sulphate aerosol from a pressurised metered-dose inhaler (pMDI)

H.-J. Tong, C. Fitzgerald, P. J. Gallimore, M. Kalberer, M. K. Kuimova, P. C. Seville, A. D. Ward and F. D. Pope

Individual micron-sized solid particles from a Salamol® pharmaceutical inhaler are stably captured in air using an optical trap for the first time. Raman spectroscopy of the levitated particles allows online interrogation of composition and deliquescent phase change within a high humidity environment that mimics the particle's travel from inhaler to lung.


A simple microfluidic dispenser for single-microparticle and cell samples

A. Kasukurti, C. D. Eggleton, S. A. Desai, D. I. Disharoon and D. W. M. Marr

Non-destructive isolation of single-cells has become an important need for many biology research laboratories; however, there is a lack of easily employed and inexpensive tools. Here, we present a single-particle sample delivery approach fabricated from simple, economical components that may address this need. In this, we employ unique flow and timing strategies to bridge the significant force and length scale differences inherent in transitioning from single particle isolation to delivery. Demonstrating this approach, we use an optical trap to isolate individual microparticles and red blood cells that are dispensed within separate 50 μl droplets off a microfluidic chip for collection into microscope slides or microtiter plates.


Wednesday, October 22, 2014

Effect of N-ethylmaleimide, chymotrypsin, and H2O2 on the viscoelasticity of human erythrocytes: Experimental measurement and theoretical analysis

Yin-Quan Chen, Chih-Wei Chen, Yu-Li Ni, Yu-Shan Huang, Orson Lin, Shu Chien, Lanping Amy Sung, and Arthur Chiou

The physiological functions of erythrocytes depend critically on their morphology, deformability, and aggregation capability in response to external physical and chemical stimuli. The dynamic deformability can be described in terms of their viscoelasticity. We applied jumping optical tweezers to trap and stretch individual red blood cells (RBCs) to characterize its viscoelasticity in terms of the Young's modulus and viscosity by analyzing the experimental data of dynamic deformation using a 2-parameter Kelvin solid model.
The effects of three chemical agents (N -ethylmaleimide, Chymotrypsin, and Hydrogen peroxide) on RBC's mechanical properties were studied by comparing the Young's modulus and viscosity of RBCs with and without these chemical treatments. Although the effects of each of these chemicals on the molecular structures of RBC may not be exclusive, based on the dominant effect of each chemical, we attempted to dissect the main contributions of different constituents of the RBC membrane to its viscosity and elasticity.


Probing fluid flow using the force measurement capability of optical trapping

Namsoon Eom, Victoria Stevens, A. Bruce Wedding, Rossen Sedev, Jason N. Connor

Interest in microfluidics is rapidly expanding and the use of microchips as miniature chemical reactors is increasingly common. Microfluidic channels are now complex and combine several functions on a single chip. Fluid flow details are important but relatively few experimental methods are available to probe the flow in confined geometry. We use optical trapping of a small dielectric particle to probe the fluid flow. A highly focused laser beam attracts particles suspended in a liquid to its focal point. A particle can be trapped and then repositioned. From the displacement of the trapped particle away from its equilibrium position one estimates the external force acting on the particle. The stiffness (spring constant) of the optical trap is low thus making it a sensitive force measuring device. Rather than using the optical trap to position and release a particle for independent velocimetry measurement, we map the fluid flow by measuring the hydrodynamic force acting on a trapped particle. The flow rate of a dilute aqueous electrolyte flowing through a plastic microchannel (W × H × L = 5 mm × 0.4 mm × 50 mm) was mapped using a small silica particle (1 μm diameter). The fluid velocity profile obtained experimentally is in very good agreement with the theoretical prediction. Our flow mapping approach is time efficient, reliable and can be used in low-opacity suspensions flowing in microchannels of various geometries.


Manifestations of geometric phase and enhanced spin Hall shifts in an optical trap

Basudev Roy, Nirmalya Ghosh, Ayan Banerjee, Subhasish Dutta Gupta and Soumyajit Roy

The spin orbit interaction (SOI) of light has been the focus of recent research due to the fundamental consequences and potential applications in diverse systems, ranging from inhomogeneous anisotropic media to engineered plasmonics and metamaterial strutures. Here, we demonstrate perhaps one of the simplest means to study SOI and the spin Hall shift (SHS) using a standard Gaussian TEM00 beam in an optical trap. Our system exploits the versatility and interference generated in a stratified medium to control and manipulate SOI and transfer the resulting angular momentum to optically trapped microparticles. We show that even such a simple setup can lead to an order of magnitude enhancement in the SHS compared to the subwavelength shifts typically obtained. Importantly, this leads to the generation of doughnut-like mode structures from a fundamental Gaussian beam, as well as controlled rotation of mesoscopic particles using a linearly polarized Gaussian beam that lacks intrinsic angular momentum. The local optical torque leading to rotation of the particles is a direct measure of the local spin angular momentum (SAM) density of the field. Our measurement is the first experimental demonstration of using a probe particle to measure the SAM density for nonparaxial fields.


Monday, October 20, 2014

A biomechanical mechanism for initiating DNA packaging

Haowei Wang, Samuel Yehoshua, Sabrina S. Ali, William Wiley Navarre and Joshua N. Milstein

The bacterial chromosome is under varying levels of mechanical stress due to a high degree of crowding and dynamic protein–DNA interactions experienced within the nucleoid. DNA tension is difficult to measure in cells and its functional significance remains unclear although in vitro experiments have implicated a range of biomechanical phenomena. Using single-molecule tools, we have uncovered a novel protein–DNA interaction that responds to fluctuations in mechanical tension by condensing DNA. We combined tethered particle motion (TPM) and optical tweezers experiments to probe the effects of tension on DNA in the presence of the Hha/H-NS complex. The nucleoid structuring protein H-NS is a key regulator of DNA condensation and gene expression in enterobacteria and its activity in vivo is affected by the accessory factor Hha. We find that tension, induced by optical tweezers, causes the rapid compaction of DNA in the presence of the Hha/H-NS complex, but not in the presence of H-NS alone. Our results imply that H-NS requires Hha to condense bacterial DNA and that this condensation could be triggered by the level of mechanical tension experienced along different regions of the chromosome.


Decoupled and simultaneous three-dimensional imaging and optical manipulation through a single objective

Arran Curran, Simon Tuohy, Dirk G. A. L. Aarts, Martin J. Booth, Tony Wilson, and Roel P. A. Dullens

The combination of optical manipulation and three-dimensional imaging is a central technique in fields ranging from medicine to physics. Using the objective lens simultaneously for optical trapping and imaging, however, inherently confines the trapping and imaging planes to the same focal plane. Here, we combine remote refocusing microscopy and optical trapping to optically decouple the imaging and trapping planes, achieving aberration-free three-dimensional imaging and simultaneous, decoupled optical trapping without the need for feedback or aberration corrections. We demonstrate our approach by directly imaging the flow field around optically trapped spheres in three dimensions. Due to its compatibility with other imaging and optical manipulation techniques, our approach is relevant to the wide range of fields that combine imaging and optical manipulation, such as physical chemistry, cell biology, and soft matter.


Friday, October 17, 2014

Beam configuration proposal to verify that scattering forces come from the orbital part of the Poynting vector

Manuel I. Marqués
In this Letter, the optical forces on an electric dipole generated by a beam made up of two circularly polarized Hermite–Gaussian modes have been analyzed. When the intensity of the two modes is not the same, the mechanical action of the scattering force is completely different from the behavior of the Poynting vector. The dynamics of resonant metallic nanoparticles under this field have been calculated by means of Langevin molecular dynamic simulations. This configuration could be useful to experimentally verify how radiation pressure on a Rayleigh particle is due to the transfer of linear momentum coming solely from the orbital part of the Poynting vector.


Microspherical photonics: Sorting resonant photonic atoms by using light

Alexey V. Maslov and Vasily N. Astratov

A method of sorting microspheres by resonant light forces in vacuum, air, or liquid is proposed. Based on a two-dimensional model, it is shown that the sorting can be realized by allowing spherical particles to traverse a focused beam. Under resonance with the whispering gallery modes, the particles acquire significant velocity along the beam direction. This opens a unique way of large-volume sorting of nearly identical photonic atoms with 1/Q accuracy, where Q is the resonance quality factor. This is an enabling technology for developing super-low-loss coupled-cavity structures and devices.


Wednesday, October 15, 2014

How to calibrate an object-adapted optical trap for force sensing and interferometric shape tracking of asymmetric structures

Matthias Koch and Alexander Rohrbach

Optical traps have shown to be a flexible and powerful tool for 3D manipulations on the microscale. However, when it comes to sensitive measurements of particle displacements and forces thorough calibration procedures are required, which can be already demanding for trapped spheres. For asymmetric structures, with more complicated shapes, such as helical bacteria, novel calibration schemes need to be established. The paper describes different methods of how to extract various calibration parameters of a tiny helical bacterium, which is trapped and tracked in shape by scanning line optical tweezers. Tiny phase differences of the light scattered at each slope of the bacterium are measured by back focal plane interferometry, providing precise and high bandwidth information about fast deformations of the bacterium. A simplified theoretical model to estimate the optical forces on a chain like structure is presented. The methods presented here should be of interest to people that investigate optical trapping and tracking of asymmetric particles.


Tuesday, October 14, 2014

Trapping particles using waveguide-coupled gold bowtie plasmonic tweezers

Pin-Tso Lin, Heng-Yi Chu, Tsan-Wen Lu and Po-Tsung Lee

We propose and demonstrate a trapping configuration integrating coupled waveguides and gold bowtie structures to form near-field plasmonic tweezers. Compared with excitation from the top, waves coupled through the waveguide can excite specific bowties on the waveguide and trap particles precisely. Thus this scheme is more efficient and compact, and will assist the circuit design on a chip. With lightning rod and gap effects, the gold bowtie structures can generate highly concentrated resonant fields and induce trapping forces as strong as 652 pN W−1 on particles with diameters as small as 20 nm. This trapping capability is investigated numerically and verified experimentally with observations of the transport, trapping, and release of particles in the system.


Site Specific Supramolecular Heterogeneous Catalysis by Optically Patterned Soft Oxometalate - Porous Organic Framework (SOM-POF) Hybrid on a Chip

Preethi Thomas, Cuiying Pei, Basudev Roy, Subhrokoli Ghosh, Santu Das, Ayan Banerjee, Teng Ben, Shilun Qiu and Soumyajit Roy
We have designed a supramolecularly bound multi-component catalytic material based on a soft oxometalate (SOM) and a porous organic framework (POF) material which shows high catalytic conversion efficiency. We have also used this material for site directed supramolecular heterogeneous catalysis with yield even higher than in the bulk, and with micron-level precision by controllably depositing the material on a glass substrate, making a reactor chip, using a thermo-optical tweezers. Various SOM-POF composites have been prepared in dispersion phase and patterned using thermo-optic tweezers and their catalytic activities have been compared with a benchmark molecular catalyst [PMo12]. This work can lead to further explorations for hybrid materials formed out of well defined molecular level precursors which can be controllably micro-patterned in order to catalyze targeted reactions simultaneously.


Strong DNA deformation required for extremely slow DNA threading intercalation by a binuclear ruthenium complex

Ali A. Almaqwashi, Thayaparan Paramanathan, Per Lincoln, Ioulia Rouzina, Fredrik Westerlund and Mark C. Williams

DNA intercalation by threading is expected to yield high affinity and slow dissociation, properties desirable for DNA-targeted therapeutics. To measure these properties, we utilize single molecule DNA stretching to quantify both the binding affinity and the force-dependent threading intercalation kinetics of the binuclear ruthenium complex Δ,Δ-[μ‐bidppz‐(phen)4Ru2]4+ (Δ,Δ-P). We measure the DNA elongation at a range of constant stretching forces using optical tweezers, allowing direct characterization of the intercalation kinetics as well as the amount intercalated at equilibrium. Higher forces exponentially facilitate the intercalative binding, leading to a profound decrease in the binding site size that results in one ligand intercalated at almost every DNA base stack. The zero force Δ,Δ-P intercalation Kd is 44 nM, 25-fold stronger than the analogous mono-nuclear ligand (Δ-P). The force-dependent kinetics analysis reveals a mechanism that requires DNA elongation of 0.33 nm for association, relaxation to an equilibrium elongation of 0.19 nm, and an additional elongation of 0.14 nm from the equilibrium state for dissociation. In cells, a molecule with binding properties similar to Δ,Δ-P may rapidly bind DNA destabilized by enzymes during replication or transcription, but upon enzyme dissociation it is predicted to remain intercalated for several hours, thereby interfering with essential biological processes.


Extended linear detection range for optical tweezers using image-plane detection scheme

Faegheh Hajizadeh, S Masoumeh Mousavi, Zeinab S Khaksar and S Nader S Reihani

Ability to measure pico- and femto-Newton range forces using optical tweezers (OT) strongly relies on the sensitivity of its detection system. We show that the commonly used back-focal-plane detection method provides a linear response range which is shorter than that of the restoring force of OT for large beads. This limits measurable force range of OT. We show, both theoretically and experimentally, that utilizing a second laser beam for tracking could solve the problem. We also propose a new detection scheme in which the quadrant photodiode is positioned at the plane optically conjugate to the object plane (image plane). This method solves the problem without need for a second laser beam for the bead sizes that are commonly used in force spectroscopy applications of OT, such as biopolymer stretching.


Flow effects in the laser-induced thermal loading of optical traps and optofluidic devices

B. del Rosal, C. Sun, Y. Yan, M.D. Mackenzie, C. Lu, A. A. Bettiol, A.K. Kar, and D. Jaque

Flow effects on the thermal loading in different optofluidic systems (optical trap and various microfluidic channels) have been systematically explored by using dye-based ratiometric luminescence thermometry. Thermal images obtained by fluorescence microscopy demonstrate that the flow rate plays a key role in determining both the magnitude of the laser-induced temperature increment and its spatial distribution. Numerical simulations were performed in the case of the optical trap. A good agreement between the experimental results and those predicted by mathematical modelling was observed. It has also been found that the dynamics of thermal loading is strongly influenced by the presence of fluid flow.


Monday, October 13, 2014

Selective plasmonic trapping in periodic gold polygon tetramers

Jiao Xie, Li Wang, Zhongwei Liao, Yingzhou Huang, Shunbo Li, Shuxia Wang, Weijia Wen

Highly bounding light at metal surface by localized surface plasmon resonance (LSPR) improves the optical trapping of nanoparticles, which is called plasmonic trapping. Since LSPR is high related to the geometry of metal structures, the construction of metal nanostructure is extremely significant in the nano-trapping. In this work, the plasmonic trapping of dielectric nanoparticles in periodic gold polygon tetramers is investigated through finite-difference time-domain (FDTD) method. The simulation results of electric field distribution and the corresponding optical force indicate the number of side is quite important to the trapping efficiency that the square tetramers is obviously superior to other ones with more sides. However, this efficiency difference is also related to the size of nanoparticle that it is more sensitive to the smaller nanoparticles. Furthermore, the results also figure out not only trapping efficiency but also the trapping position is greatly influenced by the wavelength of trapping light in the same gold polygon tetramers. All our results open a way to selectively trap nanoparticles with required size at appointed positions, which has extensive application prospects in manipulation of nanoparticles in solution.


Laser induced surface stress on water droplets

Neng Wang, Zhifang Lin, and Jack Ng

Laser induced stress on spherical water droplets is studied. At mechanical equilibrium, the body stress vanishes therefore we consider only the surface stress. The surface stress on sub-wavelength droplets is slightly weaker along the light propagation direction. For larger droplets, due to their light focusing effect, the forward stress is significantly enhanced. For a particle roughly 3 micron in radius, when it is excited at whispering gallery mode with Q ∼ 104 by a 1 Watt Gaussian beam, the stress can be enhanced by two orders of magnitude, and can be comparable with the Laplace pressure.


Single and Multiple Microparticle Trapping Using Non-Gaussian Beams From Optical Fiber Nanoantennas

Decombe, J., Mondal, S.K. ; Kumbhakar, D. ; Pal, S.S. ; Fick, J.

Optical trapping of dielectric microparticles is reported using an optical tweezers based on two original chemically etched fiber nanoantenna. The nanoantenna converts Gaussian beam into nondiffracting type quasi-Bessel beam, which is used in trapping microparticles. Stable trapping in three distinct positions is observed for an antenna distance of 32.5 $mu$m and for light powers as low as 1.3 mW. Optical trapping properties are studied by applying Boltzmann statistics to the particle position fluctuations. Harmonic trapping potentials with trap stiffness of 3.5 pN $mu$ m$^{-1}$ are observed. The FDTD simulation results on the antenna optics are also included to understand the trapping mechanism.


Simple PD Control Scheme for Robotic Manipulation of Biological Cell

Cheah, C.C.; Li, X.; Yan, X.; Sun, D.

In most manipulation techniques for optical tweezers, openloop controllers are developed to move the laser source without consideration of the dynamic interaction between the cell and the manipulator of laser source. This paper presents a simple PD control scheme for manipulation of cell using optical tweezers. We formulate a closed-loop setpoint control problem for optical tweezers and show that simple control law is effective for closed-loop manipulation, taking into consideration of the dynamic interaction between the laser beam and the cell. The use of closed-loop feedback control helps to enhance the trapping and also reduces the possibility of photodamage. The setpoint controller is also extended to a region reaching controller, where the desired objective is generalised to a region. Though the overall dynamics that involves the interaction between the cell and the manipulator is a fourth-order system, the proposed controllers do not require the use of acceleration and its derivative or the construction of any observer. Experimental results are presented to illustrate the performance of the proposed controllers.


Relativistic ponderomotive forces in the field of intense laser radiation

A. J. Castillo, V. P. Milant’ev

The motion of a relativistic charged particle in the presence of the field of high-power laser radiation represented in the form of a Gaussian beam of arbitrary mode is analyzed. The vector potential of the radiation field is expanded in terms of a small parameter (the ratio of the wavelength to the Gaussian beam waist). A specific feature of averaging with respect to the phases of the high-mode Gaussian beams is demonstrated. The averaged equations for the motion of particle and a general expression for the ponderomotive relativistic force for the circularly polarized radiation are derived. It is demonstrated that relativistic effects suppress the averaged action of high-power laser radiation on the particle.


Trapping sub-micron Size Particles in Holographic Optical Tweezers

G Dwivedi, A Gupta, M Shukla, S Kanaujia, S Yede and J T Andrews

Trapping of sub-micron size particles is of interest to the biological community as well as to nanoelectronic research and industry. We have employed spatially modified Gaussian beam to generate narrow optical traps within diffraction limitation. A spatial light modulator is addressed with the spatial frequencies of the required optical traps. The inverse Fourier transform is obtained at the trap plane of the optical tweezers. We have demonstrated the trapping of sub-micron particles in multiples traps, patterned numerically which is addressed to a spatial light modulator. The trap is vary stable and the particles are trapped for more than 120 seconds.


Wednesday, October 8, 2014

Direct measurement of the mechanical work during translocation by the ribosome

Tingting Liu, Ariel Kaplan, Lisa Alexander, Shannon Yan, Jin-Der Wen, Laura Lancaster, Charles E Wickersham, Kurt Fredrick, Harry Noller, Ignacio Tinoco Jr, Carlos J Bustamante

A detailed understanding of tRNA/mRNA translocation requires measurement of the forces generated by the ribosome during this movement. Such measurements have so far remained elusive and, thus, little is known about the relation between force and translocation and how this reflects on its mechanism and regulation. Here, we address these questions using optical tweezers to follow translation by individual ribosomes along single mRNA molecules, against an applied force. We find that translocation rates depend exponentially on the force, with a characteristic distance close to the one-codon step, ruling out the existence of sub-steps and showing that the ribosome likely functions as a Brownian ratchet. We show that the ribosome generates ∼13 pN of force, barely sufficient to unwind the most stable structures in mRNAs, thus providing a basis for their regulatory role. Our assay opens the way to characterizing the ribosome's full mechano–chemical cycle.


Growing dynamical facilitation on approaching the random pinning colloidal glass transition

Shreyas Gokhale, K. Hima Nagamanasa, Rajesh Ganapathy & A. K. Sood

Despite decades of research, it remains to be established whether the transformation of a liquid into a glass is fundamentally thermodynamic or dynamic in origin. Although observations of growing length scales are consistent with thermodynamic perspectives, the purely dynamic approach of the Dynamical Facilitation (DF) theory lacks experimental support. Further, for vitrification induced by randomly freezing a subset of particles in the liquid phase, simulations support the existence of an underlying thermodynamic phase transition, whereas the DF theory remains unexplored. Here, using video microscopy and holographic optical tweezers, we show that DF in a colloidal glass-forming liquid grows with density as well as the fraction of pinned particles. In addition, we observe that heterogeneous dynamics in the form of string-like cooperative motion emerges naturally within the framework of facilitation. Our findings suggest that a deeper understanding of the glass transition necessitates an amalgamation of existing theoretical approaches.


Comparison of T-matrix calculation methods for scattering by cylinders in optical tweezers

Xiaoqiong Qi, Timo A. Nieminen, Alexander B. Stilgoe, Vincent L. Y. Loke, and Halina Rubinsztein-Dunlop

The T-matrix method, or the T-matrix formulation of scattering, is a framework for mathematically describing the scattering properties of an object as a linear relationship between expansion coefficients of the incident and scattering fields in a basis of vector spherical wave functions (VSWFs). A variety of methods can be used to calculate the T-matrix. We explore the applicability of the extended boundary condition method (EBCM) and point matching (PM) method to calculate the T-matrix for scattering by cylinders in optical tweezers and hence the optical force acting on them. We compare both methods with the discrete-dipole approximation (DDA) to measure their accuracy for different sizes and aspect ratios (ARs) for Rayleigh and wavelength-size cylinders. We determine range of sizes and ARs giving errors below 1% and 10%. These results can help researchers choose the most efficient method to calculate the T-matrix for nonspherical particles with acceptable accuracy.


Photophoretic trapping of multiple particles in tapered-ring optical field

Fengrui Liu, Zhigang Zhang, Yufeng Wei, Qingchuan Zhang, Teng Cheng, and Xiaoping Wu

We demonstrate the photophoretic trapping of more than several hundreds of absorbing particles by tapered-ring optical traps diffracted from a circular aperture. The experiments with different laser powers show the influence of air flow acting on particles. Three kinds of particles with different densities (about 1~7 g/cm3) and different shapes (spherical, non-spherical) can be trapped. The non-spherical particles (toner particles) disperse in optical field, while the spherical particles (ink droplets and iron particles) arrange as a straight line. More importantly, in the experiments of two counter-propagating tapered-ring beams, the agglomeration of particles is achieved and can help research the dynamics of aerosols.


Simultaneous three-dimensional tracking of individual signals from multi-trap optical tweezers using fast and accurate photodiode detection

Dino Ott, S. Nader, S. Reihani, and Lene B. Oddershede
Multiple-beam optical traps facilitate advanced trapping geometries and exciting discoveries. However, the increased manipulation capabilities come at the price of more challenging position and force detection. Due to unrivaled bandwidth and resolution, photodiode based detection is preferred over camera based detection in most single/dual-beam optical traps assays. However, it has not been trivial to implement photodiode based detection for multiple-beam optical traps. Here, we present a simple and efficient method based on spatial filtering for parallel photodiode detection of multiple traps. The technique enables fast and accurate 3D force and distance detection of multiple objects simultaneously manipulated by multiple-beam optical tweezers.


Monday, October 6, 2014

Rotational elastic micro joint based on helix-augmented cross-spring design for large angular movement

Cheol Woo Ha and Dong-Yol Yang

A new type of micro-joint based on an elastic design concept is proposed for large rotational movement. The proposed new 3D micro-joint was designed based on a cross-spring that has precise and reliable motion. However, the cross-spring has a limitation in the range of rotational angle. To improve the range of rotational movement, the proposed 3D micro-joint was modified with a helical structure. By adding the helical structure, the modified rotational joint can achieve large rotational movement The micro-joint was fabricated by the two-photon stereolithography process (TPS process). The micro-joint was manipulated by optical trapping force. With the same optical trapping force, the advantage of proposed cross-spring on the large rotational movement was evaluated. And the precise movement of the proposed micro-joint was evaluated by calculating the RMS error. It has been shown that the proposed 3D micro-joint has precise and reliable motion for large rotational angle.


Calibration of Optical Tweezers for In Vivo Force Measurements: How do Different Approaches Compare?

Yonggun Jun, Suvranta K. Tripathy, Babu R.J. Narayanareddy, Michelle K. Mattson-Hoss, Steven P. Gross
There is significant interest in quantifying force production inside cells, but since conditions in vivo are less well controlled than those in vitro, in vivo measurements are challenging. In particular, the in vivo environment may vary locally as far as its optical properties, and the organelles manipulated by the optical trap frequently vary in size and shape. Several methods have been proposed to overcome these difficulties. We evaluate the relative merits of these methods and directly compare two of them, a refractive index matching method, and a light-momentum-change method. Since in vivo forces are frequently relatively high (e.g., can exceed 15 pN for lipid droplets), a high-power laser is employed. We discover that this high-powered trap induces local temperature changes, and we develop an approach to compensate for uncertainties in the magnitude of applied force due to such temperature variations.


A Label-Free Untethered Approach to Single-Molecule Protein Binding Kinetics

Ahmed A. Al Balushi and Reuven Gordon

Single molecule approaches provide rich real-time dynamics of molecular interactions that are not accessible to ensemble measurements. Previous single molecule studies have relied on labeling and tethering, which alters the natural state of the protein. Here we use the double-nanohole (DNH) optical tweezer approach to measure protein binding kinetics at the single molecule level in a label-free, free-solution (untethered) way. The binding kinetics of human serum albumin (HSA) to tolbutamide and to phenytoin are in quantitative agreement with previous measurements, and our single-molecule approach reveals a biexponential behavior characteristic of a multistep process. The DNH optical tweezer is an inexpensive platform for studying the real-time binding kinetics of protein–small molecule interactions in a label-free, free-solution environment, which will be of interest to future studies including drug discovery.


Quantification of Topological Coupling between DNA Superhelicity and G-quadruplex Formation

Sangeetha Selvam, Deepak Koirala, Zhongbo Yu, and Hanbin Mao

It has been proposed that new transcription modulations can be achieved via topological coupling between duplex DNA and DNA secondary structures, such as G-quadruplexes, in gene promoters through superhelicity effects. Limited by available methodologies, however, such a coupling has not been quantified directly. In this work, using novel magneto-optical tweezers that combine the nanometer resolution of optical tweezers and the easy manipulation of magnetic tweezers, we found that the flexibility of DNA increases with positive superhelicity (σ). More interestingly, we found that the population of G-quadruplex increases linearly from 2.4% at σ = 0.1 to 12% at σ = −0.03. The population then rapidly increases to a plateau of 23% at σ < −0.05. The rapid increase coincides with the melting of double-stranded DNA, suggesting that G-quadruplex formation is correlated with DNA melting. Our results provide evidence for topology-mediated transcription modulation at the molecular level. We anticipate that these high-resolution magneto-optical tweezers will be instrumental in studying the interplay between the topology and activity of biological macromolecules from a mechanochemical perspective.


Thursday, October 2, 2014

An optical fiber optofluidic particle aspirator

Ganapathy S. Murugan, Mohammad Bela, Christos Grivas, Ming Ding, James S. Wilkinson and Gilberto Brambilla
A fiberized optofluidic particle trapping device based on a micro-slot fabricated in a standard single-mode optical fiber by femtosecond laser micromachining is demonstrated. While fluidic convective motions move a large number of microparticles into the slot, the optical mode propagating in the nearby optical fiber core is exploited to trap and propel the particles inside the slot, thereby facilitating their collection at one of the slot extremities. The combined effect of fluidic and optical trapping allows for the collection of particles from as far as 60 μm away from the optical trap. Application to particle and live cell trapping and propulsion is demonstrated.


Direct measurement of the dielectrophoresis forces acting on micro-objects using optical tweezers and a simple microfluidic chip

In Soo Park, Se Hee Park, Dae Sung Yoon, Sang Woo Lee and Beop-Min Kim

We constructed a reliable frequency-dependent dielectrophoretic (DEP) force measurement system based on optical tweezers and a microfluidic chip. Using this system, we directly measured the frequency-dependent DEP forces acting on polystyrene beads while varying various parameters, which were all verified by theoretical simulations. We also investigated the DEP characteristics of non-functionalized and carboxyl-functionalized polystyrene beads in solutions with different conductivities by associating the measured crossover frequencies with a theoretical DEP model. This system can be used as a quantifying tool for surface conductance assays by characterizing the DEP forces acting on micro-objects in various experimental conditions.


Mechanochemical basis of protein degradation by a double-ring AAA+ machine

Adrian O Olivares, Andrew R Nager, Ohad Iosefson, Robert T Sauer & Tania A Baker

Molecular machines containing double or single AAA+ rings power energy-dependent protein degradation and other critical cellular processes, including disaggregation and remodeling of macromolecular complexes. How the mechanical activities of double-ring and single-ring AAA+ enzymes differ is unknown. Using single-molecule optical trapping, we determine how the double-ring ​ClpA enzyme from Escherichia coli, in complex with the ​ClpP peptidase, mechanically degrades proteins. We demonstrate that ​ClpA unfolds some protein substrates substantially faster than does the single-ring ​ClpX enzyme, which also degrades substrates in collaboration with ​ClpP. We find that ​ClpA is a slower polypeptide translocase and that it moves in physical steps that are smaller and more regular than steps taken by ​ClpX. These direct measurements of protein unfolding and translocation define the core mechanochemical behavior of a double-ring AAA+ machine and provide insight into the degradation of proteins that unfold via metastable intermediates.

The Influence of Ceramic Far-Infrared Ray (cFIR) Irradiation on Water Hydrogen Bonding and its Related Chemo-physical Properties

Leung TK, Lin SL, Yang TS, Yang JC and Lin YS

The property of water is highly related to the earth's environment and climate change. The fundamental dynamical process of water is include formation and breaking of hydrogen bonds. This dynamic process, so far, is still poorly understood. We investigated weakening of the hydrogen bonds of water after ceramic Far-Infrared Ray (cFIR) irradiation and the resulting effects on physical and chemical properties of water. In this study, the Fourier transform infrared spectroscopy (FT-IR) was used to explore hydrogen bonding change of cFIR-irradiated water; in addition, capillary viscometers, Gas Chromatographs (GC), Differential Scanning Calorimetry (DSC), contact angles, Franz cells, High-Performance Liquid Chromatography (HPLC), and capillary electrophoresis analysis were used to evaluate its physical characteristics, such as viscosity, volatility, temperatures of water crystallization, surface tension, diffusion, hydrogen peroxide dissociation, solubility of solid particles, and changes in pH of acetic acid. The cFIR treated water decreased in viscosity and surface tension (contact angles), but increased in the solubility of solid particles, hydrogen peroxide dissociation, temperatures of water crystallization, and acidity of acetic acid. The weakening of water hydrogen bonds caused by cFIR irradiation is correspondent with our previous medicalbiological studies on cFIR.


A Coarse-Grained Model of Unstructured Single-Stranded DNA Derived from Atomistic Simulation and Single-Molecule Experiment

Christopher Maffeo, Thuy T. M. Ngo, Taekjip Ha, and Aleksei Aksimentiev

A simple coarse-grained model of single-stranded DNA (ssDNA) was developed, featuring only two sites per nucleotide that represent the centers of mass of the backbone and sugar/base groups. In the model, the interactions between sites are described using tabulated bonded potentials optimized to reproduce the solution structure of DNA observed in atomistic molecular dynamics simulations. Isotropic potentials describe nonbonded interactions, implicitly taking into account the solvent conditions to match the experimentally determined radius of gyration of ssDNA. The model reproduces experimentally measured force–extension dependence of an unstructured DNA strand across 2 orders of magnitude of the applied force. The accuracy of the model was confirmed by measuring the end-to-end distance of a dT14 fragment via FRET while stretching the molecules using optical tweezers. The model offers straightforward generalization to systems containing double-stranded DNA and DNA binding proteins.